Solid–liquid equilibrium of quaternary system Cd2+, K+, Na+//SO42−-H2O at 298 K

Solid–liquid equilibria of the quaternary system Cd2+, K+, Na+//SO42−-H2O at 298 K were studied by an isothermal solution saturation method. Experimental results indicate five univariant curves E1F1, E1F4, E1E2, E2F2 and E2F3; two invariant points E1 and E2; four crystallization fields; and the existence of a double salt (Na2SO4·3K2SO4) in the quaternary system. Crystallization fields of equilibrium solid phases are K2SO4 (E1F1MF4 field), Na2SO4·3K2SO4 (E1E2F3F4 field), Na2SO4·10H2O (E2F2AF3 field), and CdSO4·H2O (E2E1F1BF2 field). The composition of invariant point E1 is K2SO4, Na2SO4·3K2SO4 and CdSO4·H2O. The composition of invariant point E2 is Na2SO4·3K2SO4, Na2SO4·10H2O and CdSO4·H2O. Physico-chemical properties of solution in the quaternary system show regular changes with increased cadmium concentration, indicating that CdSO4·H2O possesses the highest solubility among the three salts. This result verifies a strong transfer of Cd ion and a potential risk of soil pollution. It further shows that the solubility of K2SO4 would be restrained when the salts coexist.